It has been known that a flow rate of a fluid flowing in a tube can be measured utilizing ultrasonic wave. A representative method utilizes a variation of a period of time required for transmission of the ultrasonic wave in the flowing fluid according to the Doppler effect. The method for measuring the flow rate comprises the steps of: preparing a flow rate-measuring system in which two or more ultrasonic transducers are arranged on a tube along a tube axis; applying a sine wave alternating energy to one ultrasonic transducer so as to generate a ultrasonic wave; transmitting the ultrasonic wave through a fluid flowing within the tube; receiving the transmitted ultrasonic wave by another ultrasonic transducer; recording the period of time required for the transmission; applying a sine wave alternating energy to the latter ultrasonic transducer so as to generate a ultrasonic wave; transmitting the ultrasonic wave through the flowing fluid in the opposite direction; receiving the transmitted ultrasonic wave by the former ultrasonic transducer; recording the period of time required for the transmission; and comparing the difference between the two transmission periods with a relationship (calibration curve) between the flow rate and the difference of transmission period which has been separately prepared using a flow rate-measuring system having the same structure and the same fluid, to determine the flow rate of the flowing fluid.
Japanese Patent Provisional Publication 10-122923 describes a flow meter utilizing a system which comprises the steps of: preparing two ultrasonic transducers mounted onto an outer surface of a tube along the tube axis; applying an alternating electric energy to one transducer to generate a ultrasonic wave; transmitting the ultrasonic wave through a fluid flowing in the tube; receiving the transmitted ultrasonic wave by another transducer; alternating the generation and receipt of the ultrasonic wave between the former transducer and the latter transducer to measure a period of time for transmission of ultrasonic wave from the upstream side to the downstream side and a period of time transmission of the ultrasonic wave from the downstream side to the upstream side; and processing the difference between the measured periods of time to determine the flow rate of the fluid flowing in the tube.
Japanese Patent Provisional Publication 10-9914 describes a flow meter utilizing a system which comprises the steps of: preparing three ultrasonic transducers mounted onto an outer surface of a tube along the tube axis; applying an alternating electric energy to the central transducer to generate a ultrasonic wave; transmitting the ultrasonic wave through a fluid flowing in the tube; receiving the transmitted ultrasonic wave by the other transducers; measuring a period of time of transmission of ultrasonic wave from the upstream side to the downstream side and a period of time transmission of the ultrasonic wave from the downstream side to the up-stream side; and processing the difference between the measured periods of time to determine the flow rate of the fluid flowing in the tube.